1. Field of the Invention
The present invention relates to a fixing unit. More specifically, the present invention relates to a fixing unit which includes a heating roller and a pressure roller, and is utilized in an imaging forming apparatus such as an electrophotographic copying machine, a laser printer, an LED printer, and etc.
2. Description of the Prior Art
In such a kind of fixing unit, in many cases, the heating roller includes a core made of aluminum, and a separating layer which is formed on a surface of the core by coating fluororesin or the like and is for making a releasability of a toner good, and the pressure roller includes a core made of iron and an elastic member layer formed on a surface of the core with silicon rubber. In some prior arts, an elastic member layer is also formed on the surface of the core of the heating roller, and the separating layer is also formed on the elastic member layer.
In a prior art fixing unit 1 shown in FIG. 16, a heating roller 2 and a pressure roller 3 are rotatably supported by bearings 4 and 5, respectively. At least one of each of bearings 4 and 5 is arranged at both ends of each of the heating roller 2 and the pressure roller 3. Then, the pressure roller 3 is brought into pressure-contact with the heating roller 2 by a coil spring 6. The coil spring 6 is arranged between the bearing 5 supporting the pressure roller 3 and a main unit frame (not shown), and applies a proper pressure-contact force between the heating roller 2 and the pressure roller 3.
In the prior art shown in FIG. 16, since the coil spring 6 is utilized, it is difficult to assemble the fixing unit, and thus, an image forming apparatus. Furthermore, in this prior art, since the pressure-contact force between the heating roller 2 and the pressure roller 3 and a nip width between the both rollers are always constant irrespective of a thickness of a recording paper, a cockle or curling occurs in a thin recording paper while a fall of a fixing strength occurs in a thick recording paper having a relatively rough surface.
Therefore, in another prior art, a specific control unit for variably setting a fixing temperature in accordance with a king (size or thickness) of the recording paper is utilized. In this prior art, the fixing temperature is lowered in a case of a thin recording paper and the fixing temperature is raised in a case of a thick recording paper, and therefore, it is possible to solve the above described problem. However, in this prior art, the specific control unit becomes to be required, and therefore, a cost of an image forming apparatus becomes high.